Submillimetre Absorption of Electrolyte Solutions

Abstract

This work is a part of a research plan of the GR4/CNRS, concerning the study of ion-ion and ion-solvent interactions in electrolyte solutions. In a first step, information have been obtained by dielectric relaxation and conductivity measurements about (i) the ionic association (ii) the perturbation of Debye rotation by ionic solvation [1]. Now, our purpose is to undertake investigations about the absorption processes in such materials which occur at times shorter than 10−12s corresponding to the submillimeter wavelength range. Previous works have already shown the interest of the far infrared (FIR) spectroscopy applied to electrolyte solutions, essentially in two directions. At first, for tetraalkylammonium halides solutions in non polar solvents, a strong absorption has been shown to arise from interionic vibrations providing a possible way to test ionic pair potential models [2]. Secondly, numerous investigations have been made about the strong absorption band ascribed to solvated cations. This band due to the short range cation-solvent interactions peaks around 400 cm−1 for Li+ and at wavenumbers lower or equal to 200 cm−1 for the other alkaline cations. In this paper, we present power absorption coefficient data a at wavenumbers lower than 320 cm−1 for lithium salts solutions in various polar solvents : tetrahydrofuran (THF), 1,3-dioxolan, 1,2-dimethoxyethane. Lithium salts solutions have been chosen in order to avoid an undesirable overlap with the solvated cation band. Such spectral measurements are very difficult with the usual FIR low power sources because these liquids are very absorbing. All polar solvents exhibit a broad, strong absorption band generally ascribed to the libration of dipoles in the cage constituted by the surrounding molecules. Its maximum is situated in the 30-100 cm−1 region. Another difficulty is we need to compare the solvent and solution spectra in order to evidence the effects of the solute. In these conditions, we have decided to combine the use of a conventional Fourier transform interferometer and a FIR spectrometer to cover the range 20-320 cm−1.